Degassing molten aluminum and its alloys



Patented Oct. 6, 1936 UNITED STATES PATENT OFFICE DEGASSING MOLTENALUMINUM AND ITS ALLOYS No Drawing.

Claims.

This invention relates to removing the deleterious eifects of gas andinsoluble oxides in aluminum and its alloys. One of the principal vobjects of the invention is the provision of means 5 for degassing andpurifying large masses of molten metal in an economical yet efficaciousmanner. Another object is to employ agents for this purpose which leavesubstantially no residue or undesirable constituent in the metal chargeafter the treatment. A further object is to degas the metal in such away as not to impair its casting qualities or its physical properties inthe solid state. Still another object is to utilize degassing agentswhich do not attack the foundry or metal handling equipment.

Molten aluminum and aluminum base alloys are known to sorb aconsiderable quantity of gas and the amount found in a particular meltdepends upon the character of metal employed, Whether it is of virgin orsecondary origin, and the conditions under which the melting is done.The eifect of gas in the metal is manifested in the cast product throughthe occurrence of blowholes and so-called pinhole porosity, and byslivers or blisters on the surface of wrought articles. When in themolten state, the metal is capable of sorbing some gas but as the liquidmetal cools and solidifies in the mold, the sorption of the gasdecreases with the result that free gas is released. The released gasimmediately accumulates and forms bubbles, some of which become trappedin the freezing metal with the resultant formation of a cavity ordiscontinuity in.

the metallic structure which is a source of weakness in the casting.

Various expedients have heretofore been employed in an effort to'rid themolten aluminum of its gas content and thus overcome the adverse eifectsof gas. Melting the charge under a vacuum has been tried and whilesuccessful on a laboratory scale in extracting gas from the metal,special equipment is required which has an extremely limited capacityfor handling large molten charges. Fused salts on the surface of themolten bath have been tried both as a means of absorbing oxide particlesas well as excluding air from the metal. Such fused salts must becarefully excluded from the stream of metal entering the mold, and theyare also likely to form a crust around the walls of the metal containerwhich must be removed from time to time. The passage of an inert oractive gas through the liquid metal has also been proposed and tried. 5Although this practice has been partially success- Application January10, 1935, Serial No. 1,199

ful under some conditions, its limitations have precluded wide adoption.

I have found that the foregoing difficulties may be obviated and themetal effectively degassed through use of a duplex treatment whichconsists of adding sodium and aluminum chloride to the molten charge.The sequence of making the additions is important in that a treatmentwith aluminum chloride should follow the introduction of sodium. Thesodium may be first added to the melt which has received no preliminarycleansing treatment, and the sodium allowed to difiuse throughout themolten charge before the aluminum chloride is added. My preferredpractice, however, is to first treat the molten bath with aluminumchloride, then add the sodium, allow it to difiuse, and finallyintroduce another charge of aluminum chloride. I have discovered that aninitial treatment with aluminum chloride not only clears the melt ofsuspended dirt particles, but that it accelerates and renders the actionof the sodium more effective. The final addition of aluminum chloridereacts to dislodge the gas and virtually clear the bath of any traces ofsodium. The sodium and aluminum chloride cooperate to degas the metal toan extent that either substance used alone is unable toaccomplish. Idesignate my treatment as a duplex process because two substances areemployed to degas the metal rather than because of the number of stepsinvolved in the use of these substances.

Although molten aluminum and aluminum base alloys generally sorb gas,this problem is particularly acute in alloys containing a substantialamount of magnesium, that is, more than about 0.5 per cent. The presenceof magnesium appears to promote the sorption of gas and the condition isespecially apparent where a large part or all of the stock charged tothe furnace is of secondary origin. Methods heretofore used in treatingaluminum and its alloys have either failed or proved to be onlypartially successful in degassing alloys containing more than 0.5 percent magnesium. Through the application of my duplex treatment, however,virtually all traces of gas are removed.

In practicing my invention, the sodium is preferably added to the moltencharge as soon as practicable after the charge has melted. In this wayan economy of time is effected since the sodium has opportunity todiffuse throughout the charge while the metal bath is being brought upto the desired pouring temperature. A period of from about 15 minutes to10 hours must in any case be allowed for the charge to stand after thesodium has been added in order to secure the proper diffusion. Thesodium also escapes more slowly at lower temperatures and thus a smallerquantity will accomplish the same result as a larger amount added at amuch higher temperature. This element may be added in metallic form oras a constituent of a rich alloy used to provide the desired ingredientsof the final product. When added in metallic form the sodium ispreferably introduced in small pieces of a pound or two in weight atintervals of several minutes rather than making the addition of thetotal amount at one time. When added as a component of a rich alloy theforegoing precautions are unnecessary and hence in many cases theintroduction of sodium in this manner is most convenient and effective.If the sodium is introduced in metallic form, I have found that aspecial sodiumizer having only a few holes is a very satisfactory meansof holding the solid sodium in the bottom of the charge until it hascompletely melted and diffused into the metal bath.

Whether the sodium is added in the elemental form or as a component of arich alloy, the melt should be allowed to stand for some time after thesodium addition to secure complete diffusion of the element throughoutthe bath. The time required to accomplish this purpose varies with thesize of the charge and the temperature. For charges of a few hundredpounds a holding period of from 15 minutes up to an hour is generallysufficient. Where several thousand pounds of metal are to be treated, aholding period of from 2 to 10 hours is usually required. During thistime the temperature of the melt should not exceed about 1475 F., thepreferable range being 1325 to 1425 F. Within this temperature range theaction of the sodium is hastened but it is not too rapid, nor is thetemperature high enough to cause excessive Ioss through burning.

The amount of sodium to be added depends upon the quantity of gas in themelt; for example, a charge of scrap metal or a magnesium-bearing alloyis likely to contain more gas than a charge of virgin stock. For mostpurposes from about 0.01 to 0.1 per cent of sodium in relation to theweight of the whole charge is sufficient. For an excessively gassedcharge, however, as much as 1 per cent may be added. The amount ofsodium remaining in the final product is very small, however, and shouldnot exceed about 0.005 per cent.

Although the sodium may be introduced into the molten charge without anypreliminary cleansing of the charge, I have found that an initialtreatment with aluminum chloride removes most of the dirt admixed withthe melt, and that the action of the sodium is thereby facilitated. Thesalt not only clears the metal of suspended dross particles but itcreates a favorable condition for the action of sodium. When exposed tothe air metallic sodium becomes coated with a film which sorbs moisturewith the result that some gas is introduced along with the sodium whenit is added to the molten charge. The presence of a minute amount ofaluminum chloride, however, appears to react with this film and forms aproduct which does not gas the metal. If, on the other hand, there is nopreliminary treatment with aluminum chloride, a longer holding period isrequired subsequent to the addition of sodium to promote the degassingprocess. The action of the two substances-does not in any case appear tobe restricted to a more mechanical agitation of the liquid metal. Thequantity of anhydrous aluminum chloride needed for the preliminarytreatment varies with the size and character of the charge, from aboutto 4 ounces of the salt per hundred pounds of charge being sufficient.The salt may be conveniently added to the melt by placing it in aluminumcapsules holding as much as one pound and submerging these in the bathby means of a sodiumizer.

Whether or not aluminum chloride is initially used to treat the moltencharge, it is essential that it be introduced after the sodium hasdiffused throughout the melt. When this has occurred the anhydrous saltis then added to the molten charge while it is in the furnace or in thepouring ladle, or some is added both in the furnace and the ladle. Thesalt serves both to complete the degassing and to reduce the sodiumcontent of the metal. If the last treatment with aluminum chloride isomitted, the metal will be only partially degassed.

The amount of aluminum chloride employed for the treatment after thesodium addition may vary between about and 4 ounces per hundred poundsof metal treated. When a portion is added in the furnace and the balancein the ladle, I prefer to add the larger portion in the furnace, so asto minimize any cooling of the metal in the ladle, which would occur ifa great number of capsules were added. In adding the salt to thefurnace, capsules or packets containing as much as one pound may beused, but in adding it to the ladies only an ounce or two should beplaced in each capsule and the capsules added one after the other,rather than attempting to introduce the entire amount at one time eitherin a single container or in several capsules.

I am aware that both sodium and aluminum chloride have been separatelyemployed in the treatment of aluminum and certain aluminum base alloys.Sodium has been extensively used to modify the structure ofaluminum-silicon alloys, and when added for this purpose no substantialtime interval is allowed to elapse between the addition of this elementand the pouring oi the alloy into the mold. To permit a marked lapse oftime to occur defeats the purpose for which the element is added and thealloy becomes under-modifled. In some other cases, however, sodium isretained as a recognized constituent of the alloy. In contrast to suchuses of this element, I introduce it for the purpose of assisting inremoving the effect of any sorbed gas, and after this has beenaccomplished there is nothing to be gained by retaining it in the alloy.Only a slight trace, if any, remains in the final product. I havefurthermore ascertained that the use of sodium alone does noteffectively degas aluminum and aluminum base alloys on a commercialscale; that is, some gas remains in the metal which is released uponsolidification. It has only been through the combined use of sodium andanother substance that I have been able to rid the metal of thedeleterious effects of gas and finely dispersed oxide particles.

The use of aluminum chloride alone has been proposed as a suitablefiuxing agent for aluminum base alloys but its utility has been confinedto the removal of dirt. According to tests which I have made, treatmentwith this substance alone fails by a large margin to entirely eliminatethe effects of gas, although it partially accomplishes this result bythe use of large amounts on small melts. The duplex treatment hereindescribed and in co-pending application Serial No. 1,200 has been theonly effective means of completely eradieating the undesirable effectsof gas on a commercial scale. While I oiler no explanation of thereaction which occurs in the metal bath. it nevertheless appears thatthe sodium and aluminum chloride mutually act upon the sorbed gas todisengage it more eifectively than when either substance is used alone.

An illustration of the efiectiveness of the duplex treatment describedhereinabove is'shown by the following example. An alloy composed ofaluminum, 4.2 per cent copper, 0.5 per cent manganese and 1.5 per centmagnesium is prone to develop blisters on heat treated sheet,particularly if scrap metal alone is used in making up the charge. Undercommercial operating conditions a large rejection of sheet is frequentlyencountered because of the presence of numerous blisters attributable tothe release of gas. A 17,000 pound charge of the above alloy in the formof scrap metal was melted under the usual conditions. Approximately ipounds of sodium were added in parcels of' about 1 to 2 pounds eachshortly after the charge became molten. The melt was then allowed'tostand for 2 hours at a temperature between 1350 and 1425 F. At the endof this time pounds of anhydrous aluminum chloride in 1 pound capsuleswere introduced into the metal. The charge was permitted tostand for 4hours longer within the same temperature range before pouring it intoingot. From about 1 to 3 ounces of aluminum chloride were added to themetal in the ladle from which each 250 pound ingot was poured. Sheetmade from these ingots and heat treated failed to show any blisters.

I have also found that the treatment is effective in eliminating gasblisters in sheet made from remelted foil scrap. Foil scrap isespecially difficult to remelt because of the large area of metalexposed to the atmosphere. In remelting such stock oxide particles areintroduced in the melt. and these particles appear to have an aillnityfor gas. Other means of treating the melt than the one outlined abovehave failed to produce blisterfree sheets. The duplex treatment isespecially useful in reclaiming metal and alloys of secondary origin.

I claim:

1. A method of degassing molten aluminum and aluminum base alloyscomprising adding thereto from about 0.01 to 1 per cent of sodium,maintaining said molten charge at a temperature below about 1475 F. from4; to 10 hours, and introducing from about /2 to 4 ounces of anhydrousaluminum chloride per hundred pounds of melt.

2. A method of degassing molten aluminum and aluminum base alloyscomprising adding sodium in the molten charge, allowing the sodium todiffuse throughout the melt, and thereafter introducing anhydrousaluminum chloride into said melt.

3. A method of degassing molten aluminum and aluminum base alloys withsodium and aluminum chloride comprising initially treating the melt withanhydrous aluminum chloride, adding less than 1 per cent of sodium,maintaining said melt at a temperature below about 1475" F. for to 10hours, and finally treating the melt with anhydrous aluminum chloride.

4. A method of degassing molten aluminum and aluminum base alloys withsodium and aluminum chloride consisting of adding to the molten chargefrom about to 4 ounces of anhydrous aluminum chloride per hundred poundsof metal treated, thereafter adding from about 0.01 to 1 per cent ofsodium and maintaining the melt at a temperature below about 1475 F. fora period of A to 10 hours, and finally repeating the treatment withanhydrous aluminum chloride.

5. A method of degassing molten aluminum base alloys containing morethan 0.5 per cent magnesium comprising adding from about 0.01 to 1 percent sodium to the melt, allowing the molten charge to stand for to 10hours at a temperature between about 1325 and 1425 F. and thereafterintroducing from about /2 to 4 ounces of anhydrous aluminum chloride perhundred pounds of metal treated.

6. A method of degassing molten aluminum and aluminum base'alloys,comprising adding thereto from about 0.01 to 0.1 per cent of metallicsodium, allowing the melt to stand until the added sodium has diffusedthroughout the mass, and thereafter introducing from about to 4 ouncesof anhydrous aluminum chloride per hundred pounds of metallic chargetreated.

7. In a method of degassing molten aluminum and aluminum base alloyswith sodium and aluminum chloride, the step of initially treating themelt with from about to 4 ounces of anhydrous aluminum chloride perhundred pounds of metal treated.

8. In a method of degassing molten aluminum and aluminum base alloyswith sodium and alu minum chloride, the steps of initially treating themelt with anhydrous aluminum chloride, adding from about 0.01 to 1 percent of sodium and repeating the treatment with aluminum chloride.

9. In a method of degassing molten aluminum and aluminum base alloyswith sodium and aluminum chloride, the step of adding from about 0.01 tol per cent sodium to the melt and maintaining said melt at a temperaturebelow about 1475 F. for a period of to 10 hours before the finaltreatment with aluminum chloride.

10. A method of degassing molten aluminum base alloys containing morethan 0.5 per cent magnesium with sodium and aluminum chloride comprisingintroducing from to 4 ounces of anhydrous aluminum chloride per hundredpounds of molten charge, thereafter adding from about 0.01 to 0.1 percent sodium, maintaining the melt below about 1475" F. for a period ofA; to 10 hours, and finally repeating the treatment with aluminumchloride.

PHILIP T. STROUP.

CERTIFICATE OF CORRECTION.

Patent No. 2,056,233. October 6, 1936.

PHILIP T. STROUP.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2.,first column, line '74, for the word "more" read mere; and that the saidLetters Patent should be read with this correction therein that the samemay conform to the record of the case in the Patent Office.

Signed and sealed this 8th day 'of December, A. D. 1956.

Henry Van Aradale (Seal) Acting Commissioner of Patents.

